The low molecular weight carbohydrates in various species of the red algal genus Hypoglossum (Delesseriaceae, Ceramiales) were analysed using HPLC, 1H and 13C-nuclear magnetic resonance spectroscopy and high-resolution mass spectrometry. All specimens contained the heteroside digeneaside which is considered as chemosystematic marker for the Ceramiales. A new HPLC method was developed for the separation and quantification of this compound, and concentrations between 131.6 mmol kg(-1) and 539.6 mmol kg(-1) DW could be measured among the species tested. In addition, during the HPLC analysis another new low molecular weight carbohydrate was detected in two species from The Philippines (H. barbatum) and Western Australia (H . heterocystideum), and its chemical structure elucidated as digalactosylglycerol applying various NMR experiments. The remaining Hypoglossum taxa lack this compound. Although digalactosylglycerol occurred in high concentrations in the range of 221.7 and 438.7 mmol kg(-1) DW in H. barbatum and H . heterocystideum, respectively, it has never been reported for any other algal species before. Therefore, to test the possible physiological function of this unusual carbohydrate as organic osmolyte, H. barbatum was treated with a range of salinities. While the digeneaside content remained almost unchanged, the digalactosylglycerol concentration strongly increased with increasing salinities from 70 mmol kg(-1) DW at 20 psu to 215 mmol kg(-1) DW at 45 psu. In conclusion, while neither published work nor the present study indicate digeneaside to play more than a minor role in osmotic acclimation, the data presented strongly support an osmotic function of digalactosylglyerol.